feat: add OpenAPI shcema to custom rest backend

[popsiclexu]

Closes #

📑 Description

add OpenAPI shcema to custom rest backend tutorial
Relative PR: k8sgpt-ai/k8sgpt#1228

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

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[PixelRobots]

Hey @popsiclexu You are missing DCO from this pull request. Can you please look at the instructions below to sign off this commit.

To sign off the last commit you made, you can use

git commit --amend --signoff

You can also automate signing off your commits by adding the following to your .zshrc or .bashrc:

git() {
  if [ $# -gt 0 ] && [[ "$1" == "commit" ]] ; then
     shift
     command git commit --signoff "$@"
  else
     command git "$@"
  fi
}